This is the description of the odometer. It indicates that it is part of the IPC, and works directly with signals from the PCM, so the concerns of compatibility seem well founded.

An update to this, the DIC may be in contact with the PCM but the trip odometer is stored in the piggyback module on the back of the mainboard on the cluster primary board.

I tested this when I swapped out the odometer modules.

True. When I read the mileage with my autotap the PCM indicated I had 65,000 miles, whereas my odometer reads 63,000 miles. So even with the original odometer, the mileage is different between the PCM'* mileage reading, and the cluster'*. the Cluster'* mileage is programmable, the PCM'* is not I believe.